The impact of an insect subject in an image is increased significantly by an appropriate background. It is often the case that the natural environment of an insect can be used effectively by careful choice of viewpoint. Leaves and other greenery can make an excellent backdrop for a close-up shot of an insect, so wherever possible move the camera to a location where the subject and background are optimally aligned.
The stems of plants, and other items that may have an unwanted colour or which create strong background lines, are best kept out of a composition as far as possible. Sometimes it is possible to gently move background growth to one side or have an assistant hold it out of shot. This is never easy because butterflies can be disturbed by the smallest movement, so stealth and patience are required. An alternative approach is to find a plant that is particularly attractive from a photographic point of view and then await a suitable butterfly.
The colour of a background is also important. Avoid situations where distracting bright white areas are incorporated in the background, and try to minimize areas of high contrast or sharp colour changes. Macro images have very limited depth of field, so it is normally the case that background features are rendered unsharp. This helps to blur unwanted variations of contrast and colour because everything tends to merge together in a featureless manner.
Achieving adequate illumination for a correct exposure whilst maintaining maximum depth of field is always a challenge, so flash is used in many typical cases. Remember that the output from a flashgun falls off with distance from the camera in accordance with the inverse square law, so elements of the background may be well underexposed. Indeed a remote background may be underexposed sufficiently to be rendered almost black in an image. One way of avoiding this is to use a second flash to balance the illumination of the background with that of the subject.